This invention relates to a method and apparatus for flexographic color printing and more particularly to a method and associated apparatus for implementing a method of wet trap printing also known as xe2x80x9cwet trappingxe2x80x9d in flexographic printing, using energy curable flexographic liquid inks.
Multicolor printing processes typically require the sequential printing of a plurality of superposed single color ink layers. When high quality image reproduction is desired, it is important to avoid a previously applied ink layer mixing with a subsequently applied ink layer. Such layer mixing typically results in undesirable color rendition.
The art has addressed this problem in a number of different ways. The simplest way to prevent undesirable color mixing is to dry each applied ink layer prior to the application of a superposed next ink layer. While this method is effective it suffers a major disadvantage of requiring complete drying after applying each ink layer. Drying takes time and energy to accomplish, and as a result, productivity is reduced and production costs increase.
In an effort to speed up the printing process, wet trapping was developed. Wet trapping is a process whereby the ink layer deposited or applied at each inking station is not dried before the next ink layer is deposited thereover to produce a coloristic or visual effect. To implement wet trapping, it is important that the tack characteristics of the superposed ink layers be different.
Wet trapping is not a serious problem in offset printing, because the viscosity of the inks used in offset printing, ranges from 20,000 to 100,000 cps. Such high viscosity inks exhibit a wide range of tack characteristics that can be used to effect wet trapping without the need to dry the ink layers between inking stations.
In recent years, a form of printing that permits printing on various kinds of substrates, varying from cardboard to polyethylene to metal, has become widely accepted. This printing method is known as flexography.
Flexography employs a resilient printing plate having raised portions, which are coated with an ink and pressed against a substrate to transfer the ink to the substrate. In flexography, ink is transferred from a reservoir to the printing plate""s raised surface through an intermediate transfer roll known in the art as an anilox roll. The anilox roll surface is covered by a plurality of tiny ink wells that fill with ink from the reservoir and transfer it to the flexographic printing plate. Obviously high quality printing requires that the flexographic printing plate surface be inked uniformly and consistently. This in turn requires that the anilox roll cells be small and that all of the anilox cells be filled each time with ink from the reservoir to substantially the same level.
Such requirement poses limitations on the fluidity or viscosity of the ink. A viscous ink will not be picked up as uniformly or consistently by the anilox roll and the flexographic printing plate surface will not be inked uniformly. The result has been that inks suitable for flexographic applications typically have viscosities under 2,000 cps, preferably less than 400 cps.
Current regulations regarding solvent emissions have resulted in the development of inks suitable for use in flexography that are energy curable. Such inks contain little or no solvent, and are fixed to the substrate not by drying but by curing via actinic radiation, such as ultraviolet light or electron beam. Their tack is very low and cannot be adequately measured with conventional instruments. Their viscosities are in the range of about 30 to 50 cps. While such viscosity range results in superior flexographic printing, energy-curable inks for flexographic applications exhibit very low tack, cannot be tack rated, and need be to cured between inking stations to prevent back transfer and mixing from the printed ink on the substrate to the inking rolls of subsequent stations. Such inter-station curing is expensive, as it requires substantial equipment modification. Such curing is also undesirable from a manufacturing stand point, as it increases the time required between the deposition of a subsequent ink layer in order to allow for curing of the previously deposited ink layer, thereby slowing down the printing process.
Wet trapping has also been proposed in flexographic printing based on the recognition that when depositing superposed multiple layers of ink, mixing will not occur if each layer is deposited over a layer having a higher viscosity than the newly deposited layer. The highest viscosity layer traps, so to speak, the second layer without mixing with or transfer of the underlying layer. However, with the range of viscosities available for flexographic printing inks, it is impractical to implement wet trapping using constantly decreasing ink viscosities for each layer that are sufficiently different from each previously applied layer viscosity in order to effect wet trapping, particularly as the number of applied layers increases. In essence, one runs out of available ink viscosities to implement wet trapping.
U.S. Pat. No. 5,690,028 attempts to solve the above mentioned problem of limited available ink viscosity range using a method of wet trapping in a multicolor printing application using energy curable inks, particularly suited for a central impression press. According to this patent, the energy curable inks are heated before being applied to a substrate, and are applied to the substrate at a temperature that is higher than the previously applied ink layer. Because the temperature of the previously applied ink layer on the substrate is cooler than the heated ink, the viscosity of the previously applied ink layer is lower than the viscosity of the applied ink. This viscosity differential causes the lower viscosity ink to unilaterally transfer onto the higher viscosity ink and prevents both back trapping and ink blending.
While this method of wet trapping achieves the desired result, it requires substantial modification to the existing printing press equipment to provide for heating units in each inking station before the ink is applied to the substrate, moreover, as the number of stations increases, so must the ink temperature in the successive inking stations. Thus, it may be necessary to apply cooling to the substrate, or the printing speed may have to be reduced, in order to prevent having to increase the ink temperature to levels that may adversely affect its properties.
There is, therefore, still a need for a method to implement wet trapping when using energy curable liquid flexographic inks where little or no modification of existing printing press equipment is required, yet still permitting high speed throughput.
According to this invention, there is provided a method for the flexographic printing of multiple superposed ink layers on a substrate using at least one energy curable ink and printing a second ink thereover without prior curing of the first printed energy curable ink.
The method for applying the multiple ink layers on a substrate to effectuate wet trapping comprises, in the following order:
(a) applying onto a substrate at least one ink layer of an energy curable liquid ink having a viscosity of less than about 4000 cps and comprised of a non reactive diluent, the applied ink layer of energy curable liquid ink having a first viscosity;
(b) evaporating at least a portion of the non-reactive diluent in the applied ink layer, thereby increasing the viscosity of the applied ink layer;
(c) applying onto the previously applied ink layer of increased viscosity, at least one non-energy curable ink layer having a viscosity lower than the increased viscosity of the previously applied ink layer; and
(d) fixing both ink layers onto said substrate.
According to this invention, there is provided a method for printing multiple ink layers on a substrate, comprising selecting a first and a second aqueous energy curable liquid flexographic inks, each containing a non-reactive diluent in an amount of less than 50 wt. %, based on the weight of the ink composition, each ink having a viscosity under 4,000 cps and preferably between about 30 and 70 cps, and sequentially applying the first and second radiant energy curable liquid flexographic inks on a substrate to form first and second ink layers having superposed portions, wherein the second ink is applied after at least a portion of the non-reactive diluent in said first ink layer has evaporated.
There is also provided according to this invention apparatus for sequentially applying multiple superposed ink layers on a substrate, at least one of the ink layers being an energy curable liquid ink, having a viscosity under 4000 cps, in a way to practice wet trapping. The apparatus comprises:
(a) a substrate path and a substrate drive for driving said substrate along said predetermined path;
(b) a plurality of ink-applying stations spaced along said predetermined path, said ink-applying stations adapted to apply an ink comprising a non-reactive diluent and having a viscosity less than 4000 cps onto said substrate; and
(c) a control system for controlling said substrate transport along said path so that a first liquid ink layer, applied onto said substrate at one of said ink stations, increases in viscosity through evaporation of at least some of said diluent from said first ink layer to a viscosity higher than a viscosity of a second ink applied over said first ink layer in a subsequent ink station spaced from said first ink station, to a viscosity sufficient to wet-trap said second liquid ink as said substrate is transported between said ink stations.
According to the present invention the steps of sequentially printing ink layers may be repeated a number of times, using multiple sequential printings of radiant energy curable inks, each time allowing some of the diluent in the printed layer to evaporate and thereby have its viscosity increase, before printing the next layer of ink.
Further in accordance with the present invention, the process of increasing the viscosity of a printed ink layer on the substrate to a second viscosity by allowing at least some of the diluent in the printed radiant energy curable ink layer to evaporate may be accelerated by the application of heat or by forcing a stream of air over the inked surface between subsequent ink applications.